1. Field of the Invention
The present invention relates to a power amplifier that can be used in digital mobile communication and mobile telephones, and more particularly to a sigma-delta modulation apparatus related to a power amplifier to which sigma-delta modulated signals can be applied and to a sigma-delta modulation power amplifier that uses the sigma-delta modulation apparatus.
2. Description of the Related Art
It is preferable for a single mobile telephone such as a smart phone to be adaptable to both the wideband CDMA (W-CDMA) and CDMA2000 communication methods in the third-generation (3G) and the long term evolution (LTE) communication method in the fourth-generation (4G). It is also preferable to cover a plurality of frequency bands to operate in many regions throughout the world. A power amplifier (PA) that uses this type of mobile telephone needs to operate on signals having different peak-to-average power ratios (PAPRs), at different frequencies, and in different operating bands. In addition, since the power amplifier usually consumes much power, it is also required to be highly efficient. Furthermore, the power amplifier is required to operate more linearly to meet standardized distortion.
The efficiency of the power amplifier has a deep relationship with linear operation. To obtain high linearity, the efficiency is lowered. That is, as the PAPR of a signal to be amplified becomes larger, higher linearity is needed and operation efficiency must be lowered. The PAPRs of signals in the above W-CDMA and CDMA2000 are about 3.5 dB, but the PAPR in SC-FCDMA in LTE is about 8 dB. If carrier aggregation (CA), standardization of which will be considered in LTE, is used for transmission from a terminal, that is, for an uplink, the PAPR changes depending on the number of frequency bands to be bound.
Different power amplifiers have been conventionally used for different applications to adapt to complex signal structures. Necessarily, this requires many power amplifiers to be mounted in a small terminal and thereby imposes strain on a request to make the mobile telephone compact.
This situation places many burdens on the entire structure of a communication LSI, so a wireless unit that can be adapted to all communication methods by using software is being studied. A power amplifier requested here is such that a single power amplifier can cover all applications independently of the communication method.
As a conventional power amplifier that meets this requirement, the study of a switching-type power amplifier is being progressed. The switching-type power amplifier is advantageous in that since the power amplifier always performs saturated operation, efficiency can be made high. However, all current signals for mobile telephones have information in the amplitude direction, amplification by a certain amplitude is not possible. Therefore, a study is being made to use signals to which sigma-delta modulation has been applied in the switching-type power amplifier. In, for example, PTL 1 “Digitally Controlled Oscillation Device” (Japanese Unexamined Patent Application Publication No. 2013-089997), there is an attempt to reduce wideband noise by optimizing the set resolution of an oscillation clock. In PTL 2 “High Frequency Amplifier and Efficiency Increasing Method” (Japanese Unexamined Patent Application Publication No. 2011-041002), an attempt to improve switching efficiency by optimizing the load of a switching power amplifier is proposed. These attempts are to transmit information in the amplitude direction by using pulse density modulation (PDM). This enables the switching power amplifier to be used without alteration, so highly efficient operation can be expected.